In the present grant proposal the individual projects are designed to examine the activity of new therapeutic agents against T. gondii, to investigate new enzymatic pathways that may prove important for design of drugs active against the parasite and to identify and characterize the genes of the organism that are responsible for drug resistance. The ultimate objectives are to identify more active and safer therapeutic agents for treatment of toxoplasmosis, particularly toxoplasmic encephalitis in AIDS patients. For each of the proposed studies, tachyzoites, cysts and bradyzoites of different strains of T. gondii will be needed in large numbers. Tachyzoites and cysts will be used for infection of mice and of mammalian cells in tissue culture for in vivo and in vitro evaluation of the activity of therapeutic agents against T. gondii. Tachyzoite and bradyzoites will also be used to investigate biochemical and metabolic pathways which may prove to be useful as targets for inactivation or inhibition by therapeutic agents. These forms of T. gondii will be provided by this Core. In addition, this Core will also provide antisera to T. gondii for use in serological and immunochemical assays and antisera to IFN-gamma for use un preparation of tachyzoites of strains of T. gondii of low virulence for mice.